The present invention relates to a method for controlling stepping motors and a disk apparatus that uses a stepping motor.
In recent years, high speed accessing performance is required for disk apparatuses to feed the pick-up to a target position on the disk quickly. A disk apparatus that uses a stepping motor as a traverse motor for feeding the pick-up is already commercialized. Since the stepping motor is rotated in units of a constant basic step angle in response to the driving pulses, it is easy to open-control a strokes for feeding the pick-up and it needs no position detecting means. When using such a stepping motor for a disk apparatus, therefore, the pick-up feeding mechanism can be simplified.
However, disk apparatuses that use such a conventional stepping motor respectively have been confronted with various problems as described below. An object of the present invention is to solve such problems and provide a method for controlling stepping motors at high speeds and very accurately in an open-control that uses no detector such as a position sensor, as well as to provide a disk apparatus that uses the above-mentioned method for controlling stepping motors.
Next, various problems that will arise in disk apparatuses that use a conventional stepping motor respectively will be described in detail.
[Problems at Driving Operation of the Conventional Stepping Motors]
Hereunder, a conventional disk apparatus and a conventional method for controlling stepping motors will be explained with reference to the attached drawings. FIG. 54 is a schematic illustration for a configuration of the conventional disk apparatus. In FIG. 54, a lens 107b is held by springs 107c and 107d above a pick-up 107a. The rotational movement of a stepping motor 107f is transmitted to the pick-up 107a via a feed screw 107e. The pick-up 107a makes a linear motion in the radial direction of a disk 107j. The disk 107j stores information on its helically-formed tracks and the rotation speed of the disk 107j is controlled by a spindle motor 1071. Error signals from the pick-up 107a are transmitted to a servo means 107g. And, the servo means 107g outputs a signal for controlling the springs 107c and 107d to the pick-up 107a so that each error signal is cleared to 0. A system controller 107i, which is connected to the servo means 107g, an interface means 107k, and the spindle motor 1071, transmits a driving command signal for feeding the pick-up 107a to the stepping motor controlling means 107h as needed. By receiving the driving command signal, the stepping motor controlling means 107h controls the stepping motor 107f.
When in recording or playing back information in or from the conventional disk apparatus, the lens 107b keeps following up the helically-formed tracks on the disk 107j and the lens 107b changes its position gradually in the radial direction of the disk 107j. The servo means 107g detects each of such displacement values of the lens 107b. When the system controller 107i detects that the lens 107b has exceeded a specified displacement value, the system controller 107i transmits a driving command signal to the stepping motor controlling means 107h. By receiving the driving command signal, the stepping motor controlling means 107h rotates the stepping motor 107f step by step at fine pitches. The stepping motor controlling means 107h moves the pick-up 107a by a fine distance in the radial direction of the disk 107j to limit the displacement of the lens 107b within a low value. Then, the stepping motor 107f is kept at rest until the lens 107b exceeds the specified displacement value again.
As a means of moving the pick-up by rotating the stepping motor step by step at fine pitches, a controlling method referred to as micro-step driving operation is well known. The conventional micro-step driving method divides the basic step angle of the stepping motor into n angles (n: an integer of 2 or over) like an optical disk apparatus disclosed, for example, in Unexamined Published Japanese Patent Application Publication No. 7-272291 and changing the driving current step by step.
Next, the conventional stepping motor controlling method will be explained.
FIG. 55 is a schematic inner configuration of a general stepping motor. In FIG. 55, a numeral 106a indicates a current flowing in an A-phase coil and 106b indicates a current flowing in a B-phase coil. A rotator 106c has some pairs of N and S magnetic poles. The number of magnetic pole pairs differ among types of stepping motors. A point P on the rotator begins rotating when the current 106a flowing in the A-phase coil and the current 106b flowing in the B-phase coil are changed together. The point P stops when the balance between the magnetic force generated from those coils and the frictional load of rotation is stabilized. Positions 106X and 106Z indicate two points of some mechanical stability points existing on the rotator. Those two points are adjacent with each other. The rotation angle from the position 106X to the position 106Z is defined as the basic step angle of the stepping motor. The position 106Y indicates one of mechanical instability points existing between the positions 106X and 106Z. To rotate the stepping motor by a micro-step, the rotator 106c must be rested at a mechanical instability position in the range of the basic step angle, as shown with the position 106Y.
Next, the current flowing in each of the A-phase and B-phase coils of the stepping motor will be explained. FIG. 56 is a wave form chart indicating the driving current of the conventional stepping motor. The wave form chart shown in FIG. 56 indicates a wave form of the current in the controlling method referred to as a 1-2-phase exciting system that divides the basic step angle of the stepping motor into two angles and rotating the stepping motor in units of a 1/2 step angle. There is also another well-known controlling method, in which the basic step angle of the stepping motor is furthermore divided into n angles (n: an integer of 2 or over) and the stepping motor is rotated in units of a 1/n step angle. To make it easier to understand the explanation here, a method for controlling stepping motors with a driving current as shown in FIG. 56 will be picked up. The method divides the basic step angle into two angles, which is the least division number in the controlling methods, each of which divides the basic step angle of the stepping motor into n angles.
In FIG. 56, the wave form 105a is a driving current wave form representing the flow rate and direction of the current flowing in the A-phase coil of the stepping motor on the time axis. The wave form 105b is a driving current wave form representing the flow rate and direction of the current flowing in the B-phase coil of the stepping motor on the time axis. In the driving current wave forms 105a and 105b, the current flowing forward is represented by a positive value and the current flowing reversely is represented by a negative value. The current wave form, when the stepping motor is rotated forward, is changed from left to right in FIG. 56. The current wave form, when the stepping motor is rotated reversely, is changed from right to left in FIG. 56. If the current state is changed from 105X to 105Z in FIG. 56, it means that the state 105Y exists between those states. In the state 105Y, only the A-phase coil shown in FIG. 55 is excited and the B-phase coil is not excited. Thus, the stepping motor can stop at the position 106Y between the positions 106X and 106Z in FIG. 55. This means that the motor can stop at 1/2 of the basic step angle of the stepping motor. Since the basic step angle of the stepping motor is divided into n angles such way, the stepping motor can be rotated step by step at fine pitches. And, when using such a stepping motor for feeding the pick-up of a disk apparatus, the pick-up can be fed in micro steps in the radial direction of the disk in a recording/playback operation.
However, the conventional stepping motor controlling method and the conventional disk apparatus have the following problems.
One of the problems is that when a stepping motor is rotated by a fine step, the pick-up is accelerated significantly. In the case of the stepping motor driving current wave form shown in FIG. 56, when the stepping motor is rotated by a fine step, the state of the stepping motor driving current is changed in steps, for example, from 105X to 105Y in a moment. At this time, a large start-up torque is generated in the stepping motor and the pick-up is accelerated suddenly. Consequently, the lens of the pick-up is shaken, causing a light spot of the laser beam to be shifted from the target track on the disk. This results in an off-track error. When the off-track value is great, data cannot be recorded correctly when in recording and when in playing back, the error rate is raised.
There is another problem that arises when the stepping motor type and/or any pick-up feeding mechanism specification is changed after a stepping motor controlling method is designed. Since the stepping motor driving current wave form is fixed, the rotation value of the stepping motor cannot be adjusted. If any design is changed as mentioned above, the balance between the torque generated in the stepping motor and the frictional load of the pick-up feeding mechanism is lost. Consequently, the rotator of the stepping motor cannot be stopped accurately at a mechanical instability position between the basic step angles, so that a big stepping motor rotation angle error occurs. Such a rotation angle error results in an error of the pick-up feeding distance.
If the pick-up keeps following up the helically-formed tracks of the disk when in recording or playing back as explained above, the lens of the pick-up is displaced gradually in the radial direction of the disk. In order to suppress the displacement of the lens as less as possible, the stepping motor must be rotated in fine steps to move the whole pick-up in fine steps in the radial direction of the disk. When a big error occurs in the pick-up feeding distance, it becomes difficult to move the pick-up to cover the displaced distance of the lens. If the pick-up is moved by a distance differently from a displaced distance of the lens, the lens which follows up the track of the disk is also displaced significantly in the pick-up. And accordingly, the focus servo and tracking servo characteristics are degraded, causing a focus jump and/or an tracking-off error.
Furthermore, there will also arise another problem that the stepping motor itself generates heat and this results in a wasteful power consumption. This is because a current keeps flowing in the coils of the stepping motor even after the stepping motor is rested.
[Problems to Occur in the Conventional Stepping Motor During Operation]
In the case that a position detecting means, such as encoder, sensor or the like is not provided in a stepping motor, the stepping motor initial status, that is, the rest position of the stepping motor rotator is unknown before the stepping motor is excited.
In addition, when the stepping motor is in a step-out during rotation, the position of the stepping motor rotator becomes unknown. The step-out means a state of abnormal rotation of the rotator of a stepping motor, when the rotation goes out of synchronism with changes of the driving signal for the stepping motor.
When the position of the rotator of the stepping motor is unknown, the stepping motor is excited, then the initial position of the rotator of the stepping motor does not always come to a stability point of excitation. And, if the initial position and a stability point are not synchronized, the rotator of the stepping motor is not rotated smoothly and it might be moved suddenly to a stability point for the excitation.
In the case that the tracking servo of the disk apparatus is performed when the rotator of the stepping motor is moved suddenly to a stability point for the excitation, the pick-up and the magnetic head of the disk apparatus are also moved suddenly in the radial direction of the disk, so that they cannot follow up the tracks of the disk, causing an off-track in some cases. When the off-track value is great, data cannot be recorded accurately when in recording, and the error rate of the playback data is raised when in playing back.
Furthermore, in the case that the focus servo of the disk apparatus is performed when the rotator of the stepping motor is moved suddenly to a stability point for the excitation, then the pick-up of the disk apparatus is moved suddenly in the radial direction of the disk. Consequently, the lens of the pick-up is moved significantly, thereby causing the focus servo operation to be unstable and recording/playback of data to be disabled.
[Track Accessing Problems in Controlling of the Conventional Stepping Motor]
Next, the conventional disk apparatus will be explained with reference to the attached drawings. FIG. 57 is a block diagram for a configuration of the conventional disk apparatus. FIG. 58 is a wave form chart indicating the relationship with respect to the time among frequency change wave form (pulse rate pattern), generated torque, and necessary torque of the conventional driving pulse when in continuous rotation of the stepping motor in order to feed the pick-up 303 fast.
In FIG. 57, the disk 301 is a recording medium provided with helically-formed information tracks. The spindle motor 302 is provided to rotate the disk 1. The pick-up 303 is moved in the radial direction of the disk 301 due to the rotation of the stepping motor 307. The pick-up 303 is provided with a lens 304.
This lens 304 can be moved both vertically and horizontally by operating magnetically a focus actuator and a tracking actuator (both not illustrated) incorporated in the pick-up 303. The focus servo means 305 drives the focus actuator so that the lens is kept away by a fixed distance from the disk 301 according to the focus error signal, which is a displacement distance from the disk 301. The tracking servo means 306 drives the tracking actuator so that the lens 304 keeps following up a given track on the disk 301 according to the tracking error signal, which is a displacement distance from the track on the disk 301.
The stepping motor 307 moves the pick-up 303. The stepping motor driving means 308 applies a driving voltage to the stepping motor 307. The pick-up position detecting means 309 detects the current position of the pick-up 303 from the address information included in the data read by the pick-up 303. The pulse counting means 310 counts the number of pulses for driving the stepping motor 307 to move the pick-up 303 from the current position detected by the pick-up position detecting means 309 to a target address entered from external. The pulse rate pattern creating means 311 generates a frequency change (pulse rate pattern) of the pulses entered to the stepping motor driving means 308 according to the number of pulses counted by the pulse counting means 310. The feed screw 312 holds the pick-up 303 movably in the radial direction of the disk 301 and transmits a rotational force of the stepping motor 307 to the pick-up 303.
Next, the operation of the conventional disk apparatus formed as explained above for moving the pick-up fast will be explained.
The lens 304 is driven by an electromagnetic actuator (not illustrated) to read information from the disk 301 via the pick-up 303. This lens 304 is controlled by the focus servo means 305 so as to be kept focused on the disk 301. In addition, the lens 304 is also controlled by the tracking servo means 306 so as to keep following up the tracks on the disk 301. When accessing a given track, at first, the pulse counting means 310 counts the number of pulses for moving the pick-up 303 from the current position detected by the pick-up position detecting means 309 to the target track.
Next, the stepping motor driving means 308 drives the stepping motor 307 at a pulse rate as shown in (a) of FIG. 58 generated by the pulse rate pattern creating means 311 to move the pick-up 303 while the operation of the tracking servo means 306 stops. After the movement, the tracking servo means 306 is restarted to record/play back information. The conventional disk apparatus is formed such way. The pulse rate pattern shown in (a) of FIG. 58 is output as explained below from the pulse rate pattern creating means 311 being comprised of a microcomputer, etc.
The pulse rate for starting up the stepping motor 307 is a frequency that can start up the stepping motor 307 without causing any step-out. A step-out means a state of abnormal rotation of the rotator of a stepping motor 307, caused when the stepping motor goes out of step with the input pulse rate. When the stepping motor 307 is started up, the pulse rate is raised at a fixed change rate up to a specified frequency. After a fixed pulse rate is kept for a specified time, the pulse rate is lowered symmetrically to the pulse rate pattern when it was raised, to stop the pulse output.
There is another conventional stepping motor controlling method, which is well known as a micro-step driving method in which the basic angle of the stepping motor is divided into n angles (n: an integer of 2 or over) and the positioning resolution of the stepping motor is multiplied by n.
Next, the stepping motor driving means will be explained. The above-mentioned conventional micro-step driving method is adopted for the driving means.
FIG. 59 is a schematic illustration for an internal structure of a general stepping motor. In FIG. 59, 310a indicates a voltage applied to the A-phase coil and 310b indicates a voltage applied to the B-phase coil. The rotator 310c has a plurality of pairs of N and S magnetic poles. This number of magnetic poles differ among types of stepping motors. When the voltages 310a and 310b applied to the A-phase and B-phase coils are changed, the point P on the rotator begins a rotational motion. The point P stops at a point where the balance between the magnetic force generated from the coils and the frictional load of rotation is stabilized, that is, at a mechanical stability position.
In FIG. 59, positions 310X and 310Z indicates two points of some mechanical stability points existing on the rotator 310c. Those two points are adjacent with each other. The rotation angle from the position 310X to the position 310Z is defined as the basic step angle of the stepping motor. The position 310Y indicates one of mechanical instability points existing between the positions 310X and 310Z. To rotate the stepping motor by a micro-step, the rotator 310c must be rested at a mechanical instability position in the range of the basic step angle as shown with the position 310Y.
Next, the voltage applied to each of the A-phase and B-phase coils of the stepping motor will be explained.
FIG. 60 is a wave form chart indicating the driving voltage of the conventional stepping motor. The wave form chart shown in FIG. 60 indicates a wave form of the voltage used in the controlling method referred to as a 1-2-phase exciting system that divides the basic step angle of the stepping motor into two angles and rotating the stepping motor in units of a 1/2 step angle. There is also another well-known controlling method, in which the basic step angle of the stepping motor is furthermore divided into n angles (n: an integer of 2 or over) and the stepping motor is rotated in units of a 1/n step angle. To make it easier to understand the explanation here, a method for controlling stepping motors by dividing the basic step angle into two angles will be explained hereafter. This method is the least division number, in the controlling methods, each of which divides the basic step angle of the stepping motor into n angles.
In FIG. 60, the wave form 311a is a driving voltage wave form representing the voltage applied to the A-phase coil of the stepping motor on the time axis. The wave form 311b is a driving voltage wave form representing the voltage applied to the B-phase coil of the stepping motor on the time axis. In FIG. 60, however, when the stepping motor driving voltage applied to each of the A-phase and B-phase coils is in the state 311X, the point P shown in FIG. 59 is assumed to be at the position 310X. In the same way, when the voltage is in the state 311Y, the point P is assumed to be positioned at 310Y and when the voltage is in the state 311Z, the point P is assumed to be positioned at 310Z.
The voltage wave form, when the stepping motor is rotated forward, is changed from left to right in FIG. 60. When the stepping motor is rotated reversely, the voltage wave form is changed from right to left in FIG. 60.
If the voltage state is changed from 311X to 311Z in FIG. 60, it means that the state 311Y exists between those states. In the state 311Y, only the A-phase coil shown in FIG. 59 is excited and the B-phase coil is not excited. Thus, the point P shown in FIG. 59 can be moved to the position 311Y between the positions 311X and 311Z in FIG. 59. This means that the motor can be moved to a position by 1/2 of the basic step angle of the stepping motor. Since the basic step angle of the stepping motor is divided into n angles such way, the stepping motor can be rotated step by step at fine pitches. And, when using such a stepping motor for feeding the pick-up of a disk apparatus, the pick-up can be fed in micro steps in the radial direction of the disk during a recording/playback operation or during accessing a given track.
The configurations of such the conventional stepping motor and the conventional disk apparatus have been confronted with the following problems when in accessing a given track, however.
Hereunder, one of such the conventional problems will be explained. A stepping motor as shown in (a) of FIG. 58 is driven at a pulse rate having a trapezoidal profile in shape. Since the pulse rate is raised at a fixed change rate in the initial stage of stepping motor driving, the acceleration torque of the stepping motor is fixed at that time. However, the torque characteristics of the stepping motor have a curve shown with a broken line in (b) of FIG. 58. Thus, as shown in (b) of FIG. 58, a surplus torque unnecessary for moving the pick-up exists in the initial stage of stepping motor driving. This surplus torque causes the stepping motor to vibrate during rotation. And, this vibration is transmitted to the pick-up via the feed screw, causing the controlling of the lens to be unstable. In the worst case, this vibration causes focus jumping and tracking-off errors. In addition, such a surplus torque causes a surplus current to flow in the coils and such a surplus current causes heat to be generated in the stepping motor. Those are the problems arising from the conventional stepping motor.
Next, the vibration caused by such a surplus torque in the above-mentioned conventional example will be explained in detail.
FIG. 61 is a wave form chart indicating time-series changes of the voltages applied to the A-phase and B-phase coils of the stepping motor, as well as the displacement of the rotation angle of the stepping motor. In (a) of FIG. 61, the wave forms 312a1, 312a2, and 312a3 indicate the voltages applied to the A-phase coil. In (b) of FIG. 61, the wave forms 312b1, 312b2, and 312b3 indicate the voltages applied to the B-phase coil. In (c) of FIG. 61, the wave form 312c1 indicates the rotation angle displacement of the stepping motor when the voltages of the wave forms 312a1 and 312b1 are applied to the A-phase and B-phase coils respectively. In the same way, the wave form 312c2 indicates the rotation angle displacement of the stepping motor when the voltages of the wave forms 312a2 and 312b2 are applied to the A-phase and B-phase coils respectively. The wave form 312c3 indicates the rotation angle displacement of the stepping motor when the voltages of the wave forms 312a3 and 312b3 are applied to the A-phase and B-phase coils respectively.
In (a) and (b) of FIG. 61, 312X, 312Z, and 312W indicate voltage-applied states in each of the coils of the stepping motor respectively. Each of (a) and (b) of FIG. 61 indicates a combination of voltages applied to each of the A-phase and B-phase coils in each of the voltage-applied states 312X, 312Z, and 312W.
As shown in (a) and (b) of FIG. 61, if the voltage applied to each of the A-phase and B-phase coils of the stepping motor is in the voltage-applied state 312X shown on the left end, the target rotation angle of the stepping motor is positioned as shown with the line 312X in (c) of FIG. 61. In the same way, if the voltage applied to each of the A-phase and B-phase coils of the stepping motor is in the voltage-applied state 312Z, the target rotation angle of the stepping motor is positioned as shown with the line 312Z in FIG. 61(c). And, if the voltage applied to each of the A-phase and B-phase coils of the stepping motor is in the voltage-applied state 312W shown on the left end, the target rotation angle of the stepping motor is positioned as shown with the line 312W in FIG.(c) of FIG. 61. In the voltage-applied state 312X, however, the rotation angle of the stepping motor is assumed to be stopped at 312X.
In (a) and (b) of FIG. 61, since each period in the voltage applied states 312X, 312Z, and 312W indicates a pulse cycle, the reciprocal number of this cycle is a pulse rate. When the stepping motor is driven, a difference is generated in the rotation matching with the target rotation angle of the stepping motor as shown in (a) and (b) of FIG. 61 due to the difference between the voltages applied to the A-phase and B-phase coils, that is, the difference between the generated torques. When the voltages applied to the A-phase and B-phase coils have the wave forms 312a1 and 312b2 respectively, the rotation angle of the stepping motor is vibrated significantly as shown in the wave form 312c1. This is because an excessive torque is generated with respect to the pulse rate, that is, the rotation speed of the stepping motor.
On the contrary, if the generated torque is too small, for example, when the voltages applied to the A-phase and B-phase coils have the wave forms 312a3 and 312b3 respectively, the rotation angle of the stepping motor is changed to the next voltage-applied state before the rotation angle is displaced to the target one, as shown in the wave form 312c3. Thus, the stepping motor rotation will not be synchronized with the input driving pulses. In the worst case, the stepping motor causes a step-out and it is stopped.
On the other hand, if an optimal torque is generated with respect to the pulse rate, for example, if the voltages applied to the A-phase and B-phase coils have the wave forms 312a2 and 312b2 respectively, the rotating angle of the stepping motor enters the next voltage-applied state as shown with the wave form 312c2 when the rotating angle is displaced almost to the target one. The stepping motor is thus rotated smoothly.
Next, another problem that will arise in the conventional art stepping motor when in accessing a given track will be explained.
If, when the stepping motor is driven in micro-steps as shown in FIG. 60, the positioning resolution of the stepping motor is multiplied by n, then a torque change occurs. And, as shown in FIG. 60, if both A-phase and B-phase coils are excited into the states 311X and 311Z respectively in the range of the basic step angle, a fixed voltage is applied to both A-phase and B-phase coils and the rotator is positioned at a mechanical stability point. If the A-phase and B-phase coils are excited into the state 311Y, however, a fixed voltage is applied only to the A-phase coil and the voltage of the B-phase coil thus becomes 0. And, the rotator is positioned at a mechanical instability point.
When the rotator is positioned at a mechanical stability point, which is in the range of the basic step angle, as explained above, comparatively a large torque is generated. If the rotator is positioned at a mechanical instability point, however, the torque becomes lower than that taken when the rotator is positioned in the range of the basic step angle. Such way, the generated torque differs between when the rotator is positioned in the range of the basic step angle and when it is positioned at a mechanical instability point outside the range of the basic step angle. Consequently, the vibration of the stepping motor is further increased during a movement of the pick-up. In the worst case, the stepping motor is in the loss of synchronism.
If the rotator moves the pick-up 303 (FIG. 57) to be at a mechanical instability position, only one phase is driven when the pick-up 303 stops. Thus, the torque is in low and the pick-up 303 cannot be stopped accurately.
[Problems in the Conventional Stepping Motor Driving Mechanism]
In recent years, as a mass volume of computer programs or data is expanded more and more in scale, the use of optical disks having a larger capacity respectively is widely spread as recording or supply media of software instead of conventional floppy disks. And, functions for high speed accessing of data on such optical disks are required for those disk apparatuses. In order to make accessing faster, the pick-up must be moved to a target position quickly on the optical disk. When the pick-up is accelerated/decelerated suddenly, however, problems that the rack teeth are disengaged from the thread groove of the feed screw or vibration is generated in the pick-up arise. Consequently, an accessing mechanism that can be slid stably when the pick-up is accelerated/decelerated suddenly is indispensable to make such accessing operations faster.
Next, problems that will arise in the conventional disk apparatus when the pick-up is accelerated/decelerated suddenly will be explained.
Hereunder, an embodiment of the conventional disk apparatus will be explained with reference to the attached drawings.
FIG. 62 is a perspective view of the first example of the conventional disk apparatus. In FIG. 62, a pick-up 202 provided with a lens reads/writes signals from/on a disk 201. The pick-up 202 is provided in the pick-up base 203. A traverse motor 204 moves the pick-up base 203 in the radial direction of the disk 201. A feed screw 205 is rotated by the rotation of the traverse motor 204. On the outer periphery of the feed screw 205 is formed a thread groove 215. A rack 208, fixed to the pick-up base 203, is engaged with the feed screw 205. In the rack 208 are provided a fixing portion 206 to be fixed to the pick-up base 203 and a nut portion 207 fit in the thread groove 215. On this nut portion 207 is formed teeth 223 fit in the thread groove 215. A rack spring 224 is pressing the teeth 223 against the thread groove 215.
As shown in FIG. 62, the pick-up base 203 is guided by a guiding mechanism 211 slidably in the radial direction of the disk 201. The pick-up base 203 is provided with a guide hole 212 and the first guide shaft 209 fit in the guide hole 212 is guiding the pick-up base 203 slidably in the radial direction of the disk 201. On e pick-up base 203 is also formed a guide groove 213. The second guide shaft 210 is fit in the guide groove 213 an d used to limit the rotation of the pick-up base 203 around the guide shaft 209.
FIG. 63 is a side view (a) and a top view (b) of expanded portions in the neighborhood of both feed screw 205 and rack 208.
The fixing portion 206 and the nut portion 207 of the rack 208 are connected to a plate spring 214. Usually, the plate spring 214 is formed thinner than the nut portion 207. This is because when the movement of the pick-up base 203 is blocked by something, the plate spring 214 must be bent, so that the nut portion 207 can be released from the thread groove 215.
In the conventional disk apparatus formed as explained above, when the traverse motor 204 is rotated to accelerate/decelerate the pick-up base suddenly, the first problem that the nut portion 207 of the rack 208 is disengaged from the thread groove 215 of the feed screw 205 arises.
Furthermore, when the pick-up base 203 is accelerated/decelerated suddenly, the second problem that the pick-up base 203 is vibrated significantly arises.
Next, how the first problem will arise will be explained with reference to FIG. 63.
When the rotation of the feed screw 205 is accelerated/decelerated, the response of the pick-up base 203 to the rotation is delayed due to the inertia. Consequently, the inertia working on the pick-up base 203 is applied to the nut portion 207 via the surface of the thread groove 215. At this time, the direction of the force applied to the nut portion 207 can be resolved into 3 directional components that are orthogonal to each other; the axial direction of the feed screw 205, the radial direction of the feed screw 205 at a point where the thread groove 215 is in contact with the teeth 223 of the rack, and the tangential direction. Of those 3 directional components, especially the component of the tangential direction of the feed screw acts to shift the nut portion 207 from the thread groove 215. If this shifting force is great, the nut portion 207 is twisted and the teeth 223 go off the thread groove 215 easily. This component of the tangential direction of the feed screw 205 becomes significant when the rotation of the feed screw 205 is accelerated/decelerated suddenly or the feeding value of the feed screw per rotation is increased significantly to move the pick-up 202 fast. As a result, the teeth 223 go off the thread groove 215 easily.
In the conventional rack 208 as shown in FIG. 63, the nut portion 207 is supported only by a plate spring 214 whose rigidity is low. Thus, the nut portion 207 cannot secure a rigidity enough especially to cope with the component of the force working in the tangential direction of the feed screw 205. In the conventional disk apparatus in the status mentioned above, therefore, the nut portion 207 is deformed like being twisted.
The position (P202 position) to which the nut portion 207 is moved in FIG. 63 indicates a deformed example of the nut portion 207 and it is a position to which the nut portion 207 is deformed and moved when the feed screw 205 that has stopped is rotated and accelerated suddenly in the R202 direction.
In the structure of the conventional rack 208 shown in FIGS. 62 and 63, the rigidity of the plate spring 214 is insufficient such way to cope with the force applied from the thread groove 215 to the nut portion 207. The nut portion 207 is thus twisted significantly when the rotation of the feed screw 205 is accelerated/decelerated suddenly, so that the teeth 223 are not fit in the thread groove 215 properly. Furthermore, a problem that the teeth 223 are disengaged completely from the thread groove 215 arises.
Furthermore, since the pick-up base 203 is slidable in the radial direction of the disk 201 with a weak force, a gap is formed between the pick-up base 203 and the first guide shaft 209 of the guiding mechanism 211 and between the pick-up base 203 and the second guide shaft 210 respectively. The direction of the force applied to the nut portion 207 from the thread groove 215 has 3 directional components orthogonal to each other as explained above. In addition, since the center of the gravity of the pick-up 202 is not the same position where a force is applied to the nut portion 207, a problem that the pick-up base 203 is vibrated due to the gap of the guiding mechanism 211 arises if the head base 203 is slid at a sudden acceleration/deceleration.
In such the conventional general disk apparatus, a problem that the rack 208 mentioned above is disengaged from the feed screw 205 when in a high speed accessing, as well as a problem that the pick-up base 203 is vibrated arise respectively. In order to solve those problems, there have been proposed some countermeasures.
Next, some of the representative countermeasures for those problems will be explained.
FIG. 64 is a perspective view of the conventional disk apparatus in the second example for solving the above-mentioned problem that the rack is disengaged from the feed screw. This second example is disclosed, for example, in an unexamined Published Japanese Patent Application, publication No. 5-31479. In this prior art, the same configuration items as those of the disk apparatus in the first example shown in FIG. 62 and FIG. 63 will be given the same numerals, omitting redundant explanation. Hereunder, only the differences from the first example will be explained.
As shown in FIG. 64, at both ends of the feed screw 205 provided,with the thread groove 215 is formed a ring-like groove 216. The nut portion 207 a of the rack 208 is engaged helically with the thread groove of the feed screw 205. Thus, even when a force is applied to the rack 208a from the thread groove 215 of the feed screw 205, the rack 208a is neither deformed nor disengaged from the feed screw 205. Since a ring-like groove 216 is formed at both ends of the feed screw 205 respectively, when the nut portion 207a reaches the ring-like groove 216, the nut portion 207a is disengaged from the feed screw 205. Consequently, the nut portion 207a can be prevented from being caught in the thread groove 215. In such a structure, however, the frictional load in the helically engaged portion between the nut portion 207a and the feed screw 205 may be increased by variations of machining accuracy and temperature changes. In such a case, the disk apparatus in the second example will arise a problem that the disk cannot be accessed stably.
FIG. 65 is a perspective view of the conventional disk apparatus in the third example for solving the problem that the rack is disengaged from the feed screw. This third example is disclosed, for example, in an unexamined Published Japanese Patent Application, publication No. 5-325439. In this prior art, the same configuration items as those of the conventional disk apparatus in the first and second example shown in FIGS. 62 to 64 will be given the same numerals, avoiding redundant explanation. Hereunder, therefore, only the differences from the first and second examples will be explained.
In FIG. 65, the disk apparatus in the third example is provided with a stopper 217 and this stopper 217 is used to limit the movement of the nut portion 207 in the direction for disengaging the nut portion 207 from the feed screw 205. Since this stopper 217 is provided, the nut portion 207 can be prevented from being disengaged completely from the thread groove 215 even when the feed screw 205 is rotated at a sudden acceleration/deceleration and the nut portion 207 is deformed in the direction for disengaging the nut portion 207 from the thread groove 215. As a result, it is possible for the disk apparatus in the third example to obtain an effect of solving the above-mentioned problem.
However, since the nut portion 207 is deformed by a force received from the thread groove 215 within its movable rage, the force applied to the pick-up base 203 is changed by the deformation of the nut portion 207. And, since the force applied to the pick-up base 203 is changed such way, the pick-up 203 is vibrated. Thus, the problem that the pick-up base is vibrated cannot be solved yet here.
FIG. 66 is a perspective view of the conventional disk apparatus in the fourth example for solving the above-mentioned problem that the pick-up base is vibrated. This fourth example is disclosed, for example, in an unexamined Published Japanese Patent Application, Publication No.8-279257. In this prior art, the same configuration items as those of the disk apparatus shown in FIG. 62 to FIG. 65 will be given the same numerals, avoiding redundant explanation. Hereunder, therefore, only the differences from the first to third example will be explained.
In FIG. 66, the pick-up base 203 is provided with a guide hole 212 and the first guide shaft 209 is inserted in this guide hole 212. Consequently, the pick-up 202, guided by the first guide shaft 209, can move in the radial direction of the disk 201. The pick-up base 203 is provided with a guide bearing 216 via a bearing spring 219. In the hole of this guide bearing 218 is inserted the second guide shaft 210. The rotational motion of the pick-up base 203 around the first guide shaft is thus limited. The bearing spring 219 is pressing the guide bearing 218 against the recording face of the disk 201 in the direction orthogonal to the radial direction of the disk. The guide bearing 218 is pressed against the second guide shaft 210 by this bearing spring 219, so that the vibration of the pick-up base 203 is reduced significantly when the disk is accessed fast. However, it is only in the direction horizontal to the recording face of the disk 201 and vertical to the moving direction of the pick-up base 203 that the bearing spring 219 can suppress the vibration. Consequently, the bearing spring 219 can obtain a less effect for the vibration of the pick-up base 203 in the direction vertical to the recording face of the disk 201. Because of such the configuration of the conventional disk apparatus, the pressing force of the bearing spring 219 must further be increased to suppress the vibration of the pick-up base 203. And, when the pressing force of the bearing spring 219 is increased such way, the frictional load between the second guide shaft 210 and the guide bearing 218 is also increased. The traverse motor 204 must thus be formed so as to output a larger torque.